[Kusayarou] Lamia's Bride [SachiKing]

A cocky hero braves the ancient maze to claim its treasure, only for the serpentine guardian to fall head over heels and declare he'll become her wife. Her coils hold him still as his body betrays him—breasts swell, a thick futanari cock emerges, and a scaled tail slithers free—while she pounds her seed deep into his womb to seal the monsterization. By the end, the newlywed lamias are happily entwined, dreaming of slaughtering the human invaders foolish enough to breach their dungeon.